The present invention relates to a mechanism for adjusting tension of an inked ribbon of a printer.
The thermal printer which produces printed impressions by using an inked ribbon is well known.
There are two tension applying mechanisms, one of which is provided for applying a back tension to a feeding ribbon, and the other is provided for applying a winding-up tension. The value of the tension applied to the ribbon has influence on the quality of the printing.
When the tension is too low, the ribbon wrinkles, causing printer failures in dots.
If the tension is too high, the ribbon slips and can not be fed.
Japanese Patent Application Laid Open 7-89172 discloses a mechanism for controlling tension applied to an inked ribbon to a constant value by detecting the fluctuation of the tension of the inked ribbon which is caused by the change of diameter of the rolled ribbon during the printing operation.
In the conventional system, there must be provided a sensor for detecting the ribbon tension, and tension adjusting driving mechanisms in both of the ribbon feeding side and ribbon winding-up side. Consequently, the system becomes complicated in construction.
An object of the present invention is to provide a tension adjusting mechanism which may keep the ribbon tension constant without driving mechanisms.
According to the present invention, there is provided a mechanism for adjusting tension of an inked ribbon of a printer having a pair of frames, comprising, a ribbon winding side ribbon holder rotatably supported on the frames, a ribbon supply side ribbon holder rotatably supported on the frames, guide rollers provided for guiding an inked ribbon expanded between the winding side ribbon holder and the supply side ribbon holder, at least one ribbon tension detecting plate rotatably supported on one of the frames and supporting one of the guide rollers so as to be rotated in dependency on tension of the inked ribbon, ribbon tension adjusting means responsive to angular position of the ribbon tension detecting plate for applying a load on the corresponding ribbon holder so that the tension of the inked ribbon is adjusted to a predetermined value.
The ribbon tension adjusting means comprises a brake drum provided to be rotated together with the corresponding ribbon holder, and a brake belt slidably engaged with the surface of the brake drum, a base end of the brake belt is fixed, and a movable end of the brake belt is connected to the ribbon tension detecting plate so as to be moved by the ribbon tension detecting plate in a brake belt pulling direction.
A spring is connected to the ribbon tension detecting plate so as to urge the detecting plate in the brake belt pulling direction.
The brake drum is provided to be rotated by a power source of the printer through a power cutting off device.
The power cutting off device is a differential.
In an aspect of the present invention, the power cutting off device is a friction clutch.
The differential is composed by a bevel gear device, or a planetary gear device.
These and other objects and features of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings.